1. Field of the Invention
The invention relates to open-pore biocompatible surface layers for implants and methods of producing such surface layers, and to implants having such surface layers and to the use of such surface layers, as described herein.
2. Description of the Related Art
Implants, and especially joint replacement implants, are becoming ever more important in restorative and curative medicine. In that context, in the case of cementless joint replacement implants, the mechanically stable anchoring of the implant in the bone is of prime importance and is essential for the long-term stability and tolerability of the implants. In this case it is of prime importance that the bone is presented with an optimised joint replacement implant surface so that it can optimally bond to the implant. The surface structure and/or surface coating of a joint replacement implant is/are therefore of crucial importance because they allow—or, in unfavourable cases, prevent or impede—bonding of the bone with the implant or with the implant surface.
For the production of such overgrowth-promoting surface layers, porous layers have hitherto been found to be advantageous. Some methods are known by means of which porous layers can be produced. Materials used in this instance are biocompatible materials, especially metals such as, for example, titanium. The surface layers are arranged, for the most part, on bone implants so that their long-term anchorage in the bone is improved. The porous layers can be produced, for example, by means of a sintering technique, the structures and the sintering conditions being so selected that cavities between the metal or titanium particles applied to the surface are preserved.
With regard to cementless anchoring of joint replacement implants it is possible to differentiate between two types. One type involves bone overgrowth and the other involves bone ingrowth. 
In the case of bone overgrowth, bone cells adhere to the surface. In order for a bone-implant bond of sufficient strength to be produced, the surface must have a certain degree of roughness. Examples of this are implants roughened by means of corundum blasting or vacuum plasma sprayed titanium layers, as are illustrated, for example, in FIG. 1 as prior art. FIG. 1 shows a trabecula which has grown over a blasted surface of a prosthesis composed of a Ti6Al7Nb alloy.
With regard to bone overgrowth for bonding of a bone to an implant, it should be noted that load transmission in the sense of tensile loading is possible only to a very limited extent, since in the event of tensile stress the bone cells are simply lifted off the surface.
In the case of bone ingrowth, bone grows into cavities on and/or at the surface of the implant. Examples of such surfaces are open-pore sphere or wire surfaces, which can be produced, for example, by means of a sintering technique. Examples to be widely found of the former, sphere surfaces are those referred to as bead coatings. Such a coating is shown in FIG. 2, which shows a macrograph of a bead coating of a CoCr alloy with a bone grown into it.
Nevertheless, in many cases, stable growth of bone over such an implant surface does not, however, occur. Furthermore, ingrowth into such a sphere or wire surface, enabling tensile forces also to be transmitted to a certain extent by virtue of an interlocking of the bone with the surface which occurs on a sphere or wire surface, requires distinctly more time than bone overgrowth.
Various methods with which surfaces for bone overgrowth can be produced are known from the literature.
V. Galante et al., JBJS, 53A (1971), pages 101-114 describes, for example, the sintering of a mesh of fine titanium wires onto a substrate.
The two U.S. Pat. Nos. 3,855,638 and 5,263,986 disclose a method wherein a titanium powder composed of spherical particles of various sizes is sintered onto a substrate. 
U.S. Pat. No. 4,206,516, on the other hand, uses a ground titanium hydride powder composed of angular particles, which again is sintered onto a substrate.
By means of those methods it is possible to produce open-pore layers into whose pores bones are able to grow. Those porous structures made by means of a sintering technique can be produced by controlling the sintering conditions, it being possible to obtain cavities between the titanium particles applied to the surface of the substrate.
It is furthermore possible by means of sintering to produce a skeleton of titanium from a mixture of thermally unstable position-retainers and titanium powder or of position-retainers and titanium hydride powder. Methods in that regard are described, for example, in the Patent specifications U.S. Pat. No. 5,034,186 or WO 01/19556; likewise, a method of the company Intermedics, Austin, USA entitled “Cancellous Structured Titanium” addresses such a possibility.
However, in all titanium layers produced by a sintering process of such a kind it is inherently disadvantageous that the roughness of the erstwhile titanium particles or titanium fibres is smoothed out as a result of surface diffusion. Even though the bone can grow into the pores, it is scarcely possible for the bone cells, on a microscopic scale, to gain any hold on the smooth titanium surface. That situation is illustrated in FIGS. 3 and 4, FIG. 3 showing a scanning electron micrograph of a titanium coating produced in accordance with the above-mentioned U.S. Pat. No. 4,206,516 at 50× magnification. FIG. 4 also shows a scanning electron micrograph of the same titanium coating, but at 1000× magnification. The surface smoothed out at the microscopic level by surface diffusion can clearly be seen.
That disadvantage can be avoided by dramatically reducing the time for which the titanium particles are exposed to high temperatures.
Various methods with which surfaces for bone overgrowth can be formed are known from the literature. As already mentioned, thermal spraying processes, notably vacuum plasma spraying, form a possible way of producing a rough titanium surface industrially. The process parameters can be so selected in thermal spraying that the particles of a  layer material are heated only briefly, being at most partially melted and being rapidly quenched on meeting the substrate. As a result of the method being performed in that way, the rough surface of the spray powder is preserved to some extent and results in a visible topographical roughness of the sprayed layer. Solutions in that regard for producing a rough surface are described, for example, in U.S. Pat. No. 4,542,539, or in the articles in AESCULAP, Wissenschaftliche Information 22: “Die PLASMAPORE-Beschichtung für die zementlose Verankerung von Gelenkendoprothesen” [“The PLASMAPORE coating for cementless anchoring of joint endoprostheses”] and in “Osteointegration, Oberflächen- und Beschichtungen orthopädischer Implantate für den zementfreien Einsatz” [“Osteointegration, surfaces and coatings of orthopaedic implants for cementless use”] of PI Precision Implants AG.
The gain in roughness that can be achieved with the vacuum plasma spraying method is, however, offset by a serious disadvantage, since flame-sprayed or plasma-sprayed titanium layers generally have virtually no pores that are open to the outside and they consequently prevent ingrowth of the bone per se. Nor has it been possible for those disadvantages to be overcome by the methods mentioned in the above-mentioned references. An example of a titanium layer applied by means of vacuum plasma spraying is illustrated in FIG. 5. That Figure shows a metallographic microsection through a vacuum plasma sprayed titanium layer as a scanning electron micrograph at 100× magnification. The roughness of the titanium layer applied to the substrate can be seen clearly.
In addition, other methods are known which produce, by chemical or electrochemical means, a rough surface over which bone can grow better. The methods involved are etching or anodising methods, which are described, for example, in P.-I. Branemark et al., DE 300 74 46 C3 or in S. Steinemann et al., U.S. Pat. No. 5,456,723.
Whereas the techniques thus far presented were geared either to bone overgrowth or to bone ingrowth, there are recent developments which allow the two cementless types of anchoring in the bone to be combined. Such a surface is known, for example, under the tradename Hedrocel® or “Trabecular Metal”. Hedrocel® is a surface by means of which anchoring of the bone is possible. The production of that surface is, however, very elaborate and expensive. Starting with a highly porous foam of carbon glass, an open-pore tantalum structure is obtained by depositing tantalum onto a carbon glass framework in a chemical vapour deposition (“CVD”) process. To apply that porous structure to a surface of a bone implant, a further CVD  process is required, in which the processing temperature is more than 900° C. which may lead to impairment of the structure of the substrate, that is, of the load-bearing implant. The two-step production process of the Hedrocel® surface is described in the specifications of R.B. Kaplan, U.S. Pat. No. 5,282,861 and R.C. Cohen et al., U.S. Pat. No. 6,063,442.
A further method, which is described in European Patent Application EP 1 449 544 A1, is directed to subsequently roughening the surface of sintered particles. That roughening operation is performed by means of a wet-chemical etching process, and therefore the production of the roughened surface again results in a two-step method, namely previous application of sintered particles to a substrate and subsequent wet-chemical etching. That method also holds the danger that undesired, and indeed often dangerous, residues from the etching process will remain in the pores of the applied surface layer.
Other technologies are aimed at additionally providing porous layers such as those produced, for example, by a sintering technique with a very thin substance that promotes bone overgrowth. That substance is frequently hydroxyapatite, that is, the mineral constituent of bone. A method of that kind is described, for example, in U.S. Pat. No. 5,279,831, with application to surfaces that have already been made porous being explicitly claimed therein. U.S. Pat. No. 6,426,114 describes the application of such a coating by means of a sol-gel process. As a general rule, the hydroxyapatite layers so obtained are less than 1 μm thick and can therefore be applied to any desired surface, including, therefore, a porous surface.
In accordance with the above, it can be stated in summary that there have hitherto been many different attempts at producing, on a joint replacement implant, a surface that is structured in a satisfactory manner for the ingrowth of bone. It has, however, hitherto been possible merely                to produce open-pore coatings into which the bone can grow but which, in the sub-micrometer range, do not provide any topographical stimuli for osteoblast  adhesion and consequently for the overgrowth of bone in a manner which is rapid or better than the prior art;        to make available a surface method which provides a roughness of a few micrometers and a sub-micrometer structure for better adhesion of the osteoblasts, but those surfaces do not have an open pore structure into which the bone could grow;        to produce coatings having a high degree of roughness in the region of a few tens of micrometers, which have a limited degree of porosity but which again do not actually have a suitable, i.e. sub-micrometer, structure;        to produce open-pore coatings into which the bone can grow and whose surface has sub-micrometer roughness but which do not have a sufficiently high degree of macro-roughness and which consequently cannot “grab onto” the bone.        